ESSnuSBplus Setup Overview

• ESSnuSBplus Setup is an advanced neutrino facility that integrates a high-power accelerator with multiple beamlines and state-of-the-art detectors.
• It features coordinated components like the ESS linac-based superbeam, LEMNB, and LEnuSTORM, enabling precise flux monitoring and few-percent systematic uncertainties.
• The innovative detector suite, with far water Cherenkov tanks and near detectors, supports high-sensitivity CP violation studies and sterile neutrino searches at the second oscillation maximum.

The ESSnuSBplus (ESSnuSB⁺) setup is an advanced extension to the European Spallation Source neutrino Super Beam (ESSnuSB) project. It is designed as a precision, low-energy neutrino cross-section and oscillation complex, integrating multiple neutrino beamlines and state-of-the-art detector systems. ESSnuSB⁺ aims to deliver world-leading measurements of leptonic CP violation and detailed neutrino–nucleus cross sections by leveraging the exceptionally high-power ESS linear accelerator and optimized detector suites. The program also incorporates facilities for new-physics searches, short-baseline oscillation studies, and systematic control at the percent level (Fanourakis et al., 21 Jan 2025, ESSnuSB et al., 15 Jan 2026, Collaboration et al., 2023).

1. Accelerator Architecture and Beam Generation

ESSnuSB⁺ builds on major upgrades to the ESS linac, deploying a high-power H⁻ ion source, a chopper system, and a new accumulator ring. Key parameters include:

• Linac output: Kinetic energy $E_p = 2.0$–$2.5\,\mathrm{GeV}$, peak current $62.5\,\mathrm{mA}$, pulse length $2.86\,\mathrm{ms}$, repetition rate $14\text{–}28\,\mathrm{Hz}$, average beam power $5\,\mathrm{MW}$ (Collaboration et al., 2023, Fanourakis et al., 21 Jan 2025).
• Pulse compression: The H⁻ pulse is chopped into four $560\,\mu\mathrm{s}$ sub-pulses, each injected and accumulated, then fast extracted in $1.3\,\mu\mathrm{s}$ micro-bunches for target delivery (Collaboration et al., 2023).
• Switchyard distribution: Fast-extraction kickers dispatch pulses to four separate target/horn systems, each handling $1.25\,\mathrm{MW}$, to optimize target longevity and flexibility.
• R&D target station: A $1.25\,\mathrm{MW}$ titanium-sphere target with helium gas cooling is the prototype for full operations. Horn focusing employs $350\,\mathrm{kA}$, $100\,\mu\mathrm{s}$ pulses, optimized to focus pions in the $200\text{–}600\,\mathrm{MeV}/c$ range (Collaboration et al., 2023).

These innovations enable pulse structures and instantaneous power necessary for high-intensity, narrow-band neutrino beams with suppressed background.

2. ESSnuSBplus Beamline Complex

ESSnuSB⁺ comprises three main neutrino production facilities:

a) Main ESS linac–based Superbeam:

• Generates a high-intensity, predominantly $\nu_\mu$ ($\bar\nu_\mu$) beam from $\pi^+$ ($\pi^-$) decays, focused and sign-selected by dual magnetic horns.
• Typical flux composition at 100 km: $97.6\,\%\,\nu_\mu$, $1.7\,\%\,\bar\nu_\mu$, $0.67\,\%\,\nu_e$, $0.03\,\%\,\bar\nu_e$ in $\nu$ mode (Collaboration et al., 2023).
• Neutrino energy spectrum is broadly peaked near $0.3$–$0.4\,\mathrm{GeV}$, with rapid suppression beyond $0.6\,\mathrm{GeV}$.

b) Low-Energy Monitored Neutrino Beam (LEMNB):

• Features an instrumented decay tunnel, with calorimeters and trackers surrounding the active region, capable of $1\%$-level flux monitoring.
• Delivers neutrinos from monitored $\pi$ and $K$ decays in the $0.2$–$0.6\,\mathrm{GeV}$ window, with sharply peaked spectra and controlled systematics (Fanourakis et al., 21 Jan 2025).

c) Low-Energy nuSTORM (LEnuSTORM):

• Employs a compact muon storage ring ($p_\mu\sim300\,\mathrm{MeV}/c$; $C_\text{ring}\approx150\,\mathrm{m}$), storing $\mu^-$ or $\mu^+$ for decay-in-flight to yield well-understood $\nu_e$, $\bar\nu_\mu$ spectra (Fanourakis et al., 21 Jan 2025).
• Delivers fluxes at $L=50\,\mathrm{m}$ of order $$10^{12}\text{--}10^{13}/(\mathrm{cm}^2\,\mathrm{yr})$$.

All beamlines are co-located at the ESS site (Lund, Sweden), enabling synchronized operation and systematic intercomparison.

3. Detector Systems and Configurations

ESSnuSB⁺ incorporates a hierarchical detector suite, designed for both near and far applications:

Far Detector:

• Two water Cherenkov tanks, each $78\,\mathrm{m}\times78\,\mathrm{m}$, $\sim270\,\mathrm{kt}$ fiducial mass, $30\%$ PMT coverage (Collaboration et al., 2023).
• Sited at Zinkgruvan mine, $360\,\mathrm{km}$ (second oscillation maximum), $\approx1\,\mathrm{km}$ underground, with active and passive cosmic-ray suppression.
• Achieves energy resolution $\sigma_E/E\simeq15\%/\sqrt{E[\mathrm{GeV}]}$, angular resolution $O(1^\circ)$, background rejection (NC $\pi^0$ mis-ID $<1\%$).

Near Detector Complex:

• Comprised of a $0.75$–$1\,\mathrm{kt}$ water Cherenkov detector, a Super-Fine-Grained Detector (SFGD-$\sim1\,\mathrm{t}$ $CH$), and an Emulsion Cloud Chamber ("$\nu$iking", $\sim1\,\mathrm{t}$ $H_2O$) (Collaboration et al., 2023, Burgman et al., 2021).
• Delivers $>10^6$ $\nu_\mu$ CC and $\sim10^4$ $\nu_e$ CC events/year in WC, providing $<3\%$ flux normalization, with flavor ID $\epsilon(e\rightarrow e)=46.3\%$, $\text{misID}(\mu\rightarrow e)=1.6\%$ (Burgman et al., 2021).
• Energy reconstruction to $<5\%$ for lepton tracks in SFGD; emulsion for hadronic and MEC topologies.

LEMNB and LEnuSTORM Detectors:

• Cylindrical water Cherenkov ("LEMMOND"): $5\,\mathrm{m}$ diameter, $10\,\mathrm{m}$ length, $200\,\mathrm{t}$ total mass ($150\,\mathrm{t}$ fiducial), $25\%$ SiPMT coverage ($QE=25\%$), energy threshold $5\,\mathrm{MeV}$ (Fanourakis et al., 21 Jan 2025).
• Placed at $L=50$–$500\,\mathrm{m}$, measures cross sections and monitors $\nu$ fluxes with $O(10^4)$ CC events/year per tonne.

Detector	Location / Baseline	Mass (fiducial)	Technology/Role
WC (far)	Zinkgruvan/360 km	2 × 270 kt	CPV, oscillations, atmospheric neutrinos
WC (near)	ESS site/0.2–0.5 km	0.75–1 kt	Flux/cross-section constraint
SFGD	ESS site/0.2–0.5 km	1 t	Tracking, energy calibration, nuclear effects
Emulsion	ESS site/0.2–0.5 km	1 t	Topology (MEC), hadronic system
LEMMOND	LEMNB/LEnuSTORM	150 t	$\nu$ cross sections, sterile $\nu$ searches

4. Oscillation Physics and Secondary Measurements

ESSnuSB⁺ is optimized to exploit the second oscillation maximum ($L=360\,\mathrm{km}$, $E_\nu\sim0.3\,\mathrm{GeV}$), where the oscillation probability

$$P_{\mu e}(E,L)\simeq\sin^2\theta_{23}\sin^22\theta_{13}\sin^2(\Delta_{31}) + J_{CP}\sin(\Delta_{21})\sin(\Delta_{31})\sin(\Delta_{32})$$

(with $$J_{CP} = \frac{1}{2}\cos\theta_{13}\sin2\theta_{12}\sin2\theta_{23}\sin2\theta_{13}\sin\delta_{CP}$$, $\Delta_{ij}\equiv\Delta m^2_{ij}L/(4E)$) is especially sensitive to $\delta_{CP}$ due to an enhancement factor $\approx2.5$ vs.\ the first maximum. Matter effects remain subdominant ($< 5\%$), minimizing theoretical systematics (Fanourakis et al., 21 Jan 2025, Collaboration et al., 2023). The far detector expects $\sim1.2\times10^4$ $\nu_e$ appearance and $3\times10^4$ $\nu_\mu$ disappearance events/year, with signal efficiency $\sim70\%$ in the QE peak region (ESSnuSB et al., 15 Jan 2026).

Sterile Neutrino and BSM Searches:

• LEMNB and LEnuSTORM enable short-baseline disappearance studies, probing $\Delta m^2\sim1\,\mathrm{eV}^2$,
• 5$\sigma$ sensitivity to LSND/MiniBooNE-parameter regions, advanced evaluation of invisible decay, scalar NSI, and decoherence scenarios (Fanourakis et al., 21 Jan 2025, Collaboration et al., 2023).

Cross Section Precision:

• $\nu$–nucleus ($^{16}$O, C) cross sections in the $0.1$–$1\,\mathrm{GeV}$ band are targeted for $3$–$5\%$ uncertainty, benefitting from controlled flux and novel beam monitoring. The generic cross-section relation is:

$$\sigma(E_\nu) = \frac{N_{\text{obs}}(E_\nu)}{\phi(E_\nu)\,N_{\text{target}}\,\epsilon_{\text{det}}}$$

where $N_{\text{obs}}$ is background-subtracted yield, $\phi$ the known flux, $N_{\text{target}}$ the number of targets, and $\epsilon_{\text{det}}$ the detection efficiency (Fanourakis et al., 21 Jan 2025).

5. Systematic Uncertainties and Performance Metrics

ESSnuSB⁺ achieves systematic control at the percent level by leveraging:

• Flux normalization: $3$–$5\%$ (main beam), $<3\%$ (ND, LEMNB), $O(1\%)$ (LEnuSTORM, monitored beams).
• Cross-section modeling: $10\%$ (QE), down to $5\%$ (RES channels).
• Detector normalization: $2$–$3\%$ (efficiencies, volumes).
• Energy scale: $1\%$ correlated uncertainty at the far detector.
• Background rejection: Intrinsic beam $\nu_e$ and NC $\pi^0$ backgrounds are suppressed below $5\%$ of signal in appearance channels (ESSnuSB et al., 15 Jan 2026, Ghosh, 2021).

Expected event rates and signal/background ratios are derived with these uncertainties, employing full MC or parametric GLoBES simulations (Ghosh, 2021). Statistical precision is dominant; e.g., near detectors attain $O(1\%)$ precision on $\sigma_{\nu_e}$ with $10^4$ events per year (Burgman et al., 2021).

6. Projected Physics Reach and Extended Program

ESSnuSB⁺ is projected to provide:

• CP violation coverage: $5\sigma$ rejection of no-CPV hypothesis in $\sim72\%$ of the allowed $\delta_{CP}$ range after $10$ years; precision on $\delta_{CP}$ better than $8$–$9^\circ$ for all values (Fanourakis et al., 21 Jan 2025, Collaboration et al., 2023).
• Atmospheric parameters: Moderate–good sensitivity to the neutrino mass ordering and atmospheric mixing parameters; limited sensitivity to $\theta_{23}$ octant (Ghosh, 2021, Agarwalla et al., 2019).
• Cross section systematics: Measured to the few-percent level, providing essential input for next-generation oscillation experiments.
• Astroparticle and non-standard physics: Atmospheric, solar, supernova, and proton-decay studies become feasible by virtue of the water Cherenkov’s high mass and low threshold. Short-baseline BSM searches are enabled by the closely integrated low-energy beamlines and multiple detector technologies (Collaboration et al., 2023).

Attribute	Value or Range
Beam power (linac)	$5\,\mathrm{MW}$
Main baseline	$360\,\mathrm{km}$ (Zinkgruvan)
Far detector mass	$2\times270\,\mathrm{kt}$ (water Cherenkov)
Near detector distance	$0.2$–$0.5\,\mathrm{km}$
Cross-section precision	$3$–$5\%$ in $0.1$–$1\,\mathrm{GeV}$
$\delta_{CP}$ precision	$<9^\circ$
Systematic normalization	$3$–$5\%$ (flux), $2$–$3\%$ (detector)

7. Significance in the Neutrino Physics Landscape

The ESSnuSB⁺ setup operationalizes a phased, systematic-reduction strategy. By co-siting multiple precisely monitored beamlines and a diverse detector suite, the program achieves:

• Maximized sensitivity to leptonic CP violation by exploiting the second oscillation maximum, where the CP-odd oscillation amplitude is enhanced by a factor $\sim2.5$ compared to the first maximum, and matter effects are suppressed (Fanourakis et al., 21 Jan 2025, Collaboration et al., 2023).
• Reduction and calibration of key systematics (flux, cross-section, detector) to levels required for the next generation of precision oscillation physics and new physics searches.
• Direct investigation of sterile neutrino anomalies, as well as non-standard interactions, invisible decay modes, and decoherence effects, leveraging the coordinated short- and long-baseline configurations.

The ESSnuSB⁺ design and its associated experimental program set a technical benchmark for precision long-baseline and short-baseline neutrino oscillation studies, cross-section measurements, and systematic control (Fanourakis et al., 21 Jan 2025, ESSnuSB et al., 15 Jan 2026, Collaboration et al., 2023).